A few days ago, I woke up unusually early in the morning but couldn't fall back to sleep (due to neck pain). I've been ramping up my jhana meditation lately, and suddenly I attained either first jhana or access concentration, could see a nimitta, and had a completely clear mind. At the same time I kind of fell asleep, but not really, and could hear that I was breathing deeply and even snoring. I wasn't awake and I wasn't asleep. The same thing happened a couple of years ago in the middle of the night. Could this be what is referred to as yoga nidra?

Classic hypnopompic lucidity phenomena: A clear nimitta or sacred geometry, body partially felt, early morning.

(I avoid traditional terms like 'yoga nidra' because they mean different things to different people, but that is usually used to refer to lucidity during deep sleep, which is quite different from what you describe.)